The present invention relates to an injection compacting composition for preparing sintered bodies of metallic powders which comprises a metallic powder and a specific organic binder. More particularly, the present invention relates to the injection compacting composition which has a good injection compactibility and shows a high shape retainability of compacts prepared therefrom without deformation when removing the binder from the compacts, and which can provide sintered bodies having no or little defects such as warpage and cracks and having a high density in high yields, and also relates to sintered bodies prepared therefrom and a process for the preparation of the sintered bodies.
Sintered bodies have been prepared by compacting a mixture of an inorganic powder with an organic binder and sintering the resulting compacts. In recent years, in the field of ceramics, ceramic products have been prepared by mixing a ceramic powder and an organic binder, imparting a plasticity to the mixture, injection-compacting the mixture to give green compacts, removing the binder from the compacts and sintering the compacts. The production of sintered ceramic products by injection molding has the features that parts of complicated shape which cannot be produced by press molding, can be mass-produced industrially.
On the other hand, in the field of metallic sintered bodies, they have been prepared for a long time by so-called powder metallurgy method, namely by mixing a metal powder with a slight amount of an organic substance as a binder, compacting the mixture by press molding, and sintering the compacts. In recent years, in order to mass-produce sintered metallic materials of complicated shape having a high density, it has been attempted to apply the injection compacting method which has been used for the production of ceramic articles, to the production of sintered metallic materials.
However, in the production of sintered metallic materials, for the reasons that (1) metal powders have a relatively large particle size, whereas ceramic powders have an average particle size as fine as below 3 .mu.m, (2) metal powders used as raw materials for the production of sintered metallic materials have in general a higher specific gravity than usual ceramic powders such as alumina powder, and (3) metal powders are poor in wettability with binders as compared with usual ceramic powders, for example, oxide ceramic powders such as alumina, even if it is tried to produce sintered metallic materials under similar conditions to those in the production of ceramics, problems are encountered, e.g. poor injection compactibility, lack of strength of green compacts and deformation of compacts in removal of the binder. Accordingly, it has been difficult to produce good sintered metallic materials as in the case of the production of ceramic products, and at times even green compacts cannot be satisfactorily prepared.
In order to solve such problems, attempts to remove the binder by a special method or under special conditions have been made, but satisfactory results have not been necessarily obtained.
Removal of the binder from compacts prepared by compacting a mixture of a metal powder and the binder by thermal decomposition has been generally conducted by embedding the compacts in a powder unreactive to the compacts, e.g. alumina or zirconia, whereby seeping out of the binder by capillary action is accelerated to shorten the time required for binder removal, and the shape retainability of compacts is raised. However, the removal of binder by such a method requires a wider space due to the use of powder medium for embedding the compacts, and also, when taking the compacts in and out from the powder medium, the surface of the compacts may be frequently scratched. Also, it is troublesome to completely remove the medium powder sticking to the surface of the compacts which have become fragile by the removal of binder, and it frequently takes time beyond expectation.
For producing sintered metallic materials by injection compacting, there are required various characteristics, e.g. a mixture of a metal powder and a binder having an excellent compactibility such as powder flowability or compacting stability, the obtained compacts having a high strength sufficient for handling, and the binder being easily removed from the compacts without causing cracking, blistering and deformation such as warpage. Accordingly, the selection of binders to be admixed with metallic powders is important.
As the organic binder used in compacting ceramic powders, there have been known, for instance, waxes and polymers such as ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), polystyrene, polyethylene, and acrylic and methacrylic polymers. These binders are also be applicable to the production of sintered metallic materials. However, these known binders have both merits and demerits and are not always satisfactory in required characteristics such as compactibility (e.g. compacting stability, mold releasing property and powder flowability in compacting inorganic powders to which the binders are added), strength of green compacts, removability of binder from green compacts by thermal decomposition prior to the sintering, shape retainability in thermal treatment for removal of binder and in sintering and amount of residual carbon.
For instance, ethylene-vinyl acetate copolymer (EVA) is superior in powder flowability and can provide green compacts having a high strength and an adequate elasticity without impairing the flowability when mixed with a metal powder. However, it is difficult to conduct the binder removal treatment without impairing the compacts when it is used in a large amount, because defects such as blister and cracks are easy to occur in removing the binder from compacts by thermal decomposition.
Similarly to EVA, the ethylene-ethyl acrylate copolymer (EEA) can also provide a high strength and an adequate elasticity to compacts without impairing the flowability when mixed with metallic powders. However, if it is used in a large quantity, blisters and cracks are markedly produced in the compacts when removing the binder by thermal decomposition. Thus it is difficult to remove the binder without impairing the compacts.
Also, polystyrene, and acrylic and methacrylic polymers (e.g. polyisobutyl methacrylate) have an excellent binding effect on metallic powders and impart a high strength to green compacts. In particular, they are effective for preventing damages of thin portions, thus imparting an excellent shape retainability to green compacts. They are also superior in thermal decomposability and are easy to be removed from the compacts. However, when they are used in a large quantity, mixtures with metallic powders are insufficient in flowability, and are easy to invite poor results in injection molding such as shortage of filling and weld line.
In order to obtain a binder having satisfactory properties, it is desirable to use these binders in combination by combining two or more of binders having different characteristics in a good balance. However, a difficulty is encountered in obtaining a homogeneous mixture by mixing such known binders, or a long time is required for the mixing, because they differ from each other in form and softening point and also because the solubility or compatibility is not satisfactory. In general, if the mixing of two or more binders is not so sufficient as to provide a homogeneous mixture, it takes a longer time to determine the compacting conditions since the flowability of the mixture is not stabilized. Also, since the uniformity of obtained compacts is impaired, the use of nonuniform binder mixture may cause cracking of sintered bodies in addition to influence on dimensional stability of compacts.
It is an object of the present invention to provide a metal powder composition which is suitable for the preparation of sintered bodies of metal powders by injection molding followed by sintering of compacts, and which can provide sintered bodies of complicated shape having a high density and no defects such as warpage, cracks and sink marks in high yields on a mass-productive industrial scale.
Another object of the present invention is to provide a sintered body of a metallic powder having no defects.
A still another object of the present invention is to provide a process for preparing a sintered metallic body wherein the removal of binder from compacts is conducted without using a powder medium.
These and other objects of the present invention will become apparent from the description hereinafter.